This invention relates to the manufacture of semiconductor light-emitting devices and, in particular, to the grading of semiconductor lasers for appropriate applications.
Semiconductor lasers currently in use usually include a narrow stripe for light emission. This stripe may be defined by a change in index of refraction across the lateral dimensions of the active layer (index guided structure) or by current confinement within the desired portion of the active layer (gain guided structure). The stripe may comprise the entire width of the active region surrounded by high bandgap material or comprise only a portion of the active region. (See, for example, Casey and Panish, Heterostructure Lasers, Part B, pp. 207-217 and 240-242, Academic Press, 1978.)
Semiconductor lasers can exhibit nonlinearities or "kinks" in their light output versus current characteristic. The presence of such kinks could disqualify the laser from use in systems requiring high accuracy. Thus, detecting kinks in the light output can be a critical step in reliability. To make matters worse, it is hypothesized that some devices may not exhibit the kink until after field installation when it becomes difficult or impossible to replace the laser. It is, therefore, highly desirable during the manufacturing process to be able to identify devices which exhibit or have the potential for exhibiting kinks in their output. This identification could be done on final device structures. However, it would also be highly beneficial to identify problems early in the fabrication stage to avoid costly processing. For example, if defects could be detected during the wafer state of processing, subsequent metallizations and cleaving could be avoided for unsuitable devices.
It has been suggested that the appearance of kinks depends upon stripe width and stripe thickness (see, e.g., Paoli, "Nonlinearities in the Emission Characteristics of Stripe-Geometry A1GaAs Double Heterostructure Junction Lasers," IEEE Journal of Quantum Electronics, Vol. QE-12, pp. 770-776 (1976), and Dutta et al, Journal of Lightwave Technology, Vol. LT-2, pp. 160-164 (1984)). In addition, other phenomena which adversely affect laser performance, such as beam wander, may be correlated with the size of the stripe. Thus, a measure of stripe width and thickness could provide a means for determining the acceptability of a laser for high reliability applications. In fact, since stripe thickness is easier to control during laser fabrication, a measure of stripe width alone could be used for grading the laser. Unfortunately, the standard method of measuring stripe width involves techniques which destroy the laser facet or could otherwise introduce damage to the laser.
Consequently, it is a primary object of the invention to provide a nondestructive method of measuring the stripe width of light-emitting structures in order to grade the reliability of the final devices.